The present invention relates to electrochemical cells and more particularly to a rechargeable cell that has an improved cathode structure.
Recent developments in high energy density cells have focused attention on anodes of alkali metals, particularly lithium, nonaqueous electrolytes and cathode-active materials of nonstoichiometric compounds. Alkali metals, particularly lithium, are used as anode-active materials because they are highly electronegative, thereby providing high energies per weight unit. Self-discharge of electrochemical cells employing alkali metals as the anode-active material is minimized by employing nonaqueous solvents which are not reducible by the highly reactive anode materials. The choice of cathode material is frequently the compromise of a number of factors. Much attention has been directed to layered nonstoichiometric compounds which are solid, significantly oxidizing, react with the ions of alkali metals without substantial structural changes, and are electronically conductive.
Although the nonstoichiometric compounds possess many of the characteristics preferred for the cathode materials, problems still remain. The current generating electrochemical reactions in electrochemical cells employing nonstoichiometric compounds as the cathode-active material are dependent upon surface area and it is difficult to prepare structurally sound cathodes without materially decreasing the accessibility of the electrolyte to the cathode-active material. Moreover, although the nonstoichiometric compounds are electronic conductors, their conductivity is not nearly as great as metallic conductors. In order to overcome these problems as well as others, it has been suggested to provide the cathode structure by incorporating a noncorroding electrochemically inactive binder such as polytetrafluoroethylene, or to improve the conductivity of the cathode structure by mixing particulate graphite with the cathode-active material. The general application of graphite and carbon materials as conductive elements is restricted since graphite has only a moderately high conductivity and since fabricated forms of these materials, such as fibers, are costly and have relatively poor mechanical properties. The choice of metallic structural members and fibers for binding and contacting the cathode-active material is restricted in high energy density, secondary electrochemical cells because thermodynamic considerations indicate that many useful metallic materials will corrode under the highly oxidizing conditions experienced during charging to the high cut-off potentials that are implicit in high energy density cells.